A person who metabolises a probe drug—the rate of which is related to the metabolising cytochrome P-450 enzyme—slower than others; a person can be a poor metaboliser of one probe drug, and an extensive metaboliser of another
6 Depending upon the activity of the enzyme and the combinations of the alleles, individuals are classified as poor metabolisers (PMs), intermediate metabolisers (IMs), extensive metabolisers (EMs) and ultra-extensive metabolisers (UMs) of CYP2D6 substrates.
3) For drugs inactivated by the CYP enzymes, poor metabolisers will be slow to clear the drug and risk toxicity, while ultrafast metabolisers will clear the drug too rapidly and may not achieve therapeutic plasma concentrations (see box 1, p22).
Up to 10 percent of people of European descent, and three per cent of Asians, Africans and Maori and Pacific peoples, are poor metabolisers at CYP2D6.
Colour is thought to show your constitutional leanings: so the brown- eyed, for example, tend to be poor metabolisers of fats; the blue-eyed are liable to develop acid conditions such as arthritis; while those with mixed irises tend to suffer poor digestion.
As well as age-related changes in metabolic processes, there are segments of the population who may unknowingly be genetically poor metabolisers for particular drug metabolism pathways, with the result that drug dosage should be reduced to avoid adverse effects.